The extracellular matrix (ECM) is composed of a diverse set of macromolecules, including both proteins and polysaccharides, which form the three-dimensional environment within which cells exist in the body and constitute the space-filling material between cells. The ECM can also be organized into a sheetlike layer known as the basal lamina or basement membrane. In many regions of the body, the basement membrane lies beneath layers or tubes of epithelial cells (e.g., endothelial cells lining blood vessels) or surrounds individual cells of various types such as muscle cells, often serving to separate cell layers from one another or from adjacent connective tissue.
The ECM consists primarily of molecules that are secreted locally and assemble into a scaffold that stabilizes and supports the physical structure of cell layers and tissues. However, rather than being merely an inert substrate for cell attachment, the ECM constitutes an environment that is rich in biological information. It is recognized that the ECM, and various biomolecules associated with it (e.g., secreted locally or transported to a particular site from elsewhere), exert a significant influence on many aspects of cell behavior and phenotype, regulating processes such as migration and proliferation, influencing cell development and differentiation, and affecting cell shape and function. The structure of the ECM is, in turn, influenced by the cells within it. Not only do these cells secrete many ECM constituents, but they also help to pattern the matrix. Thus it is evident that cell-ECM interactions are of vital importance.
While a vast amount of useful biological information has been gathered from experiments performed on cells grown on traditional tissue culture substrates such as glass or plastic, there has been increasing interest in developing culture systems and materials that would more accurately reflect the native cellular environment. Such materials would have use not only for cell culture but also for tissue repair and tissue engineering, e.g., for growing cells, tissues, and/or artificial organs or for use in cell-based bioreactors for production of biomolecules.
Many previous efforts to develop such systems have involved the use of materials such as proteins and peptides obtained from animal sources. However, these materials have a number of disadvantages as compared with synthetic materials. For example, they present an increased risk for the transmission of disease. Even when harvested under supposedly sterile conditions, there is a significant risk of contamination. If animal sources are used, there is concern about immunogenicity if the materials are subsequently introduced into the human body, e.g., for tissue repair or as components in artificial organs. In addition, it can be difficult to ensure that different preparations of material have a consistent, reproducible composition. Even when it is possible to achieve consistency with respect to the known components of a material isolated from a natural source, it is hard or impossible to ensure that unknown, perhaps as-yet unidentified components that may affect cell properties, are excluded. Furthermore, in the course of harvesting, processing, and/or reconstituting these natural products they may become damaged or degraded, thus potentially reducing the fidelity with which they replicate the native cellular environment.
Another approach to the development of materials that would mimic the environment provided by the ECM is to produce various ECM constituents by recombinant DNA techniques. For example, expression constructs encoding ECM proteins can be introduced into prokaryotic or eukaryotic cells, and the protein of interest can be purified from the cells or from the medium in the case of secreted proteins. Proteins can be combined in vitro in desired ratios. While likely reducing the likelihood of disease transmission, this approach also suffers from several disadvantages. For example, whenever proteins are manufactured through a biological rather than purely synthetic process, there remains the possibility that undefined components from the culture system will be present even in highly purified preparations. In addition, purification can be time-consuming and costly and can result in protein degradation or denaturation.
Although native ECM consists largely of proteins and proteoglycans, significant efforts have been directed to development of cell culture and tissue engineering materials based on a variety of synthetic, non-amino acid based polymers. For example, aliphatic hydroesters have been widely used for various tissue engineering applications. Among the commonly used fully synthetic materials are polyglycolic acid (PGA), poly(lactic-co-glycolic acid) (PLGA), poly(ethylene glycol) (PEG), poly(propylene glycol), and various copolymers of these and other compounds. However, these materials also suffer from a number of disadvantages. For example, they form fibers with diameters on the order of tens of microns, which is significantly larger than the diameter of fibers found in native ECM. Furthermore, the methods required to introduce cells into matrices formed from such materials may not be readily compatible with physiological requirements for cell viability.
Thus there remains a need for synthetic compositions and materials for cell culture and tissue engineering purposes that would allow the creation of a cellular environment that mimics important aspects of the native cellular environment without the disadvantages associated with products derived from natural sources. For example, it would be desirable to develop a material that could provide biologically relevant stimuli to cells akin to those provided by native ECM components. For applications involving implantation into the body, there remains a particular need for such compositions and materials that elicit no or minimal immune or inflammatory response and for compositions and materials that are degradable within the body. In addition, there remains a need in the art for compositions and materials that would influence cell properties and functions in desirable ways.